Hate to say, but if you were slipping the clutch much off-road (especially in sand, with high loads and low speed) then you probably did toast it. If you've got the mechanical know-how, you might try pulling the clutch plates and sanding the friction plates, as they are likely glazed (as opposed to wearing the friction material off). You want to sand very lightly, just enough to remove the glaze. At least on dirt bikes, you can lay the bike over (gently, of course) on its side so the oil runs away from the clutch, remove the cover, do the work, and reassemble, without draining the oil. FWIW, since I am not an oil technician, I stick to purpose-specific motorcycle oils. Other oils MIGHT work, and perhaps HAVE worked, under particular circumstances. But I figure the oil techs know more than I do, and have worked out the best formula to work with the wet clutch.